The present invention relates to a matrix display device comprising a plurality of row and column address conductors, a plurality of picture elements each comprising an electro-optical display element connected in series between a row conductor and a column conductor with an associated two terminal non-linear resistance device comprising a diode ring type circuit providing first and second conduction paths in parallel between the two terminals, each of which paths exhibits a threshold characteristic and contains at least three series-connected diode elements arranged to allow current flow through the circuit in respective opposite directions.
An active matrix display device of this kind is suitable for displaying alpha-numeric or video, e.g. TV information.
Matrix display devices of the above kind using diode elements and in which the electro-optical display elements comprise liquid crystal display elements are known, although other passive electro-optical media such as electrophoretic or electrochromic materials may be used instead.
In FIG. 1 of the accompanying drawings there is shown diagrammatically two examples of the basic circuit configuration of a typical picture element and its associated row and column conductors of a known form of matrix liquid crystal display device. In these circuits, each liquid crystal display element 12, constituted by a pair of spaced electrodes with liquid crystal material therebetween, is connected in series with a diode ring circuit type of bi-directional non-linear resistance device 14, comprising in these examples a pair of diodes connected in parallel with opposing polarities, i.e. in anti-parallel, between a row, scanning, conductor 16 and a column, data, conductor 18. The two forms of circuit configurations shown are electrically equivalent and perform in the same manner. The choice between them is made purely on technological grounds.
The transmission (T)-RMS voltage (Vlc) curve of the liquid crystal material, the current (I) voltage (V.sub.R) characteristic of the diode ring circuit and the drive waveforms applied to the row and column conductors are illustrated in FIGS. 2, 3 and 4 respectively.
The purpose of the diode ring circuit is to act as a switch in series with the display element. When a given row of the display device is to be addressed the voltage applied to the row conductor concerned, illustrated by the waveform of FIG. 4a, is taken to one, Vs, of two selected voltage levels. In common with most other liquid crystal display systems, the polarity of the voltage applied across the liquid crystal display element is inverted every field (i.e. every 20 m.sec. for a PAL TV system) to prevent unwanted degradation of the liquid crystal material and generally poor image quality. Since the operation of the picture elements in the positive and negative cycles are exactly equivalent the following discussion will consider a cycle of only one polarity for simplicity.
During the "select" period ts (FIG. 4a), corresponding in the case of a TV display to a maximum of a line period, the voltage across a diode ring circuit and its associated display element (and other diode rings and display elements in the same row) causes the diode ring to operate in the charging part of the diode ring circuit characteristic indicated at C in FIG. 3. In this region the diode ring circuit current is large and the display element capacitance rapidly charges to a voltage, Vp, given by the expression: EQU Vp=Vcol-Vs-Vd, (1)
where Vcol and Vs are respectively the voltage applied to the column conductor 18 at that time and the select voltage applied to the row conductor 16, and Vd is the forward voltage drop across the diode ring circuit at the end of the charging period. Vcol is derived, in the case of a TV display, by sampling the appropriate line of the incoming video signal in accordance with known practice. At the end of the select period ts the row voltage falls to a new, lower, and constant value Vh (FIG. 4a) which is selected so that the mean voltage across the diode ring circuit during the next approximately 20 milliseconds, corresponding to the usual field period for TV display less the duration of the period ts, when the row is next addressed again with a select voltage, is minimized. In theory, assuming an ideal situation, this sustain, or hold, voltage Vh is equal to the mean of the rms saturation and threshold voltages (as shown in FIG. 2), that is: EQU Vh=(Vsat+Vth)/2. (2)
Under these conditions the maximum voltage of either polarity appearing across the diode ring circuit is equal to the peak to peak voltage on the column conductor, which in turn is equal to the difference between the rms saturation and threshold voltages Vsat and Vth. As the voltage across the diode ring circuit increases larger leakage currents flow through the diodes and vertical crosstalk appears. For a given level of display performance it is possible to derive a maximum acceptable diode voltage which is shown at Vdm in FIG. 3. This means that the display will only operate correctly if the condition: EQU Vsat-Vth&lt;Vdm (3)
is satisfied. Vdm can be controlled by using greater numbers of diodes for each display element or by varying the way in which the diodes are fabricated so that the slope of the diode I-V curve is changed. The latter approach only allows small changes to be produced so the main way in which the diode ring circuit characteristics can be matched to the liquid crystal is to increase the numbers of diode elements used until Vdm for the combination satisfies the above equation. Examples of known circuits of typical picture elements employing greater numbers of diode elements are shown in FIGS. 5a and 5b. The bidirectional non-linear resistance device shown in FIG. 5a consists of two sets of diode elements, each set having a plurality of diode elements connected in series with similar polarity for unidirectional conduction and the two sets being connected in parallel with opposing polarities. FIG. 5b shows an alternative form of circuit in effect comprising a number of series-connected diode rings each comprising a pair of diode elements connected in parallel and with opposing polarities. The two forms of circuit function in a similar manner electrically.
British Patent Specification No. 2129183A describes liquid crystal matrix display devices using diode ring circuits of the aforementioned kinds. Reference is invited to this specification for further details of the fabrication and operation of this type of display device.
Clearly, the smaller the difference between Vsat and Vth, the fewer diode elements are needed. However, a certain difference is needed to allow grey scale levels to be accurately reproduced. The use of the minimum number of diode elements is desirable for two reasons. First of all the chances of producing a faulty diode element increase as their number increases and so the yield of good displays becomes lower as the numbers increase. Secondly, for a display device operated in the transmission mode, and bearing in mind that the diode elements are usually fabricated side by side and situated adjacent an electrode of their associated display element on a substrate of the device, the effective optical transmission area of the display becomes smaller as more diode elements are used, making the display dimmer for a given backlight power.
The voltage across the display element resulting from driving in the above described manner has a waveform as depicted in FIG. 6a.
If the known forms of diode ring circuits shown in FIGS. 5a and 5b are used the effect of one diode element in the circuit becoming open circuit is that the diode ring circuit cannot conduct for one of the polarities of drive voltage and in this situation the voltage across the diplay element then has a waveform as shown in FIG. 6b. This signal has a different rms value from the correct one. The signal also has a large 25 Hz component as well as a large dc component. The combined effect of these characteristics is that the picture element has a different brightness, exhibits a 25 Hz flicker and does not respond to changes in drive signal in the same way as a normal picture element. Such a picture element is therefore seen as a visible defect in the display picture and renders the display device unacceptable for high quality display purposes. In order to alleviate this problem, British Patent Specification No. 2129183A, mentioned earlier, proposes to use two diode ring circuits for each picture element so that, in the event of one diode ring circuit being defective through an open-circuit diode element, the other diode ring circuit controls the display element as required to provide satisfactory operation. This proposal suffers from the disadvantage that the required duplication of non-linear resistance devices complicates manufacture and leads to a greater proportion of the display device's area being occupied by the non-linear resistance devices and therefore a lesser amount of active display area.